Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069; 5,593,838; and 5,603,351, all assigned to the David Sarnoff Research Center, Inc. The system uses an array of micron sized reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps comprising electrically activated electrodes within the microchannels proved the propulsive forces to move the liquid reagents within the system. One class of electrokinetic pump, which is known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and then pumped into a bottom array of reaction cells. The array may be decoupled from the assembly and removed for incubation or analysis.
The above described microfluidic pumping can be used as a printing apparatus. The chemical reagent solutions are replaced by dispersions or solutions of ink colorants. The array of reaction cells may be considered a viewable display of picture elements, or pixels, comprising mixtures of colorants having the hue of the pixel in the original scene. When contacted with the paper, the force of the paper fibers pulls the colorant from the cells and holds it in the paper, thus producing a paper print, or photograph, of the original scene.
One problem with microfluidic printing is in the excess amount of fluid being transferred to the receiving medium during printing. The reducing solution and inks can be aqueous or organic solutions or aqueous or organic dispersions. In the case of solutions, to avoid precipitation or phase separation, the concentrations of the inks are required to be below the solubility limits of the inks in the respective carrier solvent. For pigments, the concentration is dependent on the stability of the dispersion to prevent coalescence of the dispersed particulate. Typically, the concentrations for the colorant and the pigmented inks are below 10 wt % and 15 wt % respectively. Thus, for transferring a fixed amount of colorant to a receiving medium as required by the image, a large amount of carrier solvent needs also to be absorbed by the receiving medium. This increases the ink absorbing materials to be coated on the receiver as well as the cost of the receiver. It also increases the time for the inks to dry on a receiving medium after printing.